The invention relates to an electric arc cutting or welding torch with a torch head, the assembly and disassembly of which relative to the torch body is made more reliable and more robust than in the prior art.
Electric arc cutting or welding torches with a head which can be dismantled comprise two main parts which can be assembled or disassembled relative to one another, i.e. a base or torch body which is habitually secured on the structure, such as a support frame provided with a mobile beam, and a removable torch head or nose.
Examples of plasma cutting torches with a head which can be dismantled are known in particular from documents EP-A-599709, GB-A-2091594, EP-A-801882 and EP-A-941018.
The torch head contains the consumables of the torch, typically an electrode, an upstream gas diffuser, an upstream nozzle, a downstream gas diffuser and a downstream nozzle, as well as the internal circuits for distribution of the gases and the cooling liquid.
The torch body is an interface between an electric generator, the high-frequency and/or high-voltage housing which is necessary for priming of the arc, the housing for regulation of the gases, and the cooling system.
As a result, the nozzles and electric wires obtained from the different elements converge towards the torch body which contains the different inputs and outputs for fluids, i.e. cooling fluid, plasma-producing gas and protective gas, as well as the electrical connections for the cathode and the priming and control circuits.
The torch head is supplied with fluids and electric current by means of the torch body. In addition, the torch body ensures the mechanical connection of the torch head to the remainder of the electric arc cutting or welding machine.
The consumables of the torch are wear parts. They are eroded more or less rapidly when the torch is used, and need to be replaced regularly.
The implementation of an electric arc cutting or welding torch habitually involves the following steps:                fitting of the consumables in the torch head;        assembly of the torch head on the torch body;        selection of the process parameters and starting of the cutting or welding operation at the machine control panel;        stoppage of the cutting or welding operation;        disassembly of the torch head from the torch body in order to replace the worn consumables.        
The fitting and dismantling of a torch head on or from a torch body require particular precautions in order to ensure the continuity of the fluid (gas and cooling liquid) and electrical connections of the torch, i.e. the sealing of the fluid duct connections and good contact between the electrical connections.
In particular, the alignment of the torch head and the torch body is essential in order to permit connection of the head to the torch body which is not only rapid, but which also does not damage the electrical and fluid connections which project at the upstream end of the head, and which must be fitted in housings with complementary connections situated in the torch body.
In addition, in the case of incorrect assembly, an electric arc can be established between the conductive parts of the torch head and the torch body. An electric arc of this type can lead to the destruction of the torch head and body.
An assembly process is known from document FR-A-2949698 consisting of screwing the head on the torch body by means of a screwing nut. The torch head and/or the torch body additionally comprise axial and angular polarisation means which make it possible to align the fluid ducts and the electrical connections of the head and the body before screwing and tightening the nut.
However, screwing of the nut is a problematic manual operation, on completion of which the operator has no way of knowing if the torch head is correctly engaged in the torch body. In fact, during intensive use of the torch in an aggressive environment such as that which exists when cutting metal plate, i.e. projections of molten metal, generation of fumes, UV radiation, heat, etc., the threads of the tightening nut on the torch head and/or the corresponding thread on the torch body are inevitably filled with a fine metal powder which makes the tightening operation difficult. Depending on the force applied, the operator can have the impression that the torch head is correctly engaged in the body, whereas in reality the fluid sealing and/or the electrical contact at the interface between the head and the torch body is/are defective.
A plasma arc cutting torch is also known from document FR-A-2698301, wherein the torch body comprises a ball cage which cooperates with a groove provided in the periphery of the torch head.
However, because of the aggressive environment encountered in industrial production, the movement of the balls tends to deteriorate rapidly and irreversibly. Ultimately, the torch body becomes unusable and must be replaced.
The problem which is posed is thus to provide an arc cutting or welding torch, the torch head and the torch body of which can be assembled or disassembled easily manually, without encountering the aforementioned problems, in particular whilst ensuring the continuity of the fluid and electrical connections of the body and the torch head, and in a manner which is more reliable and more robust than in the prior art.